I was doing the air flow in a 90 degree bend experiment.
On the first run, I connected the manometer to the tappings on the outer wall, then I turned off the bench and changed the connections to the inner wall. Then, I turned on the air flow again.
Each time I done this I got a different reading on the manometer for the airbox pressure and the atmospheric pressure.
The air flow settings weren't changed and no change in room temperature or nothing. It was like 10 minutes difference between each run.
Why is this?